Process for the production of liquid metal from fine-grain metal oxide particles and reducing and smelting furnace for carrying out the process

ABSTRACT

In a process and an apparatus for the production of liquid metal (4) from fine-grain metal oxide particles, the particles, together with hot reducing gas, are blown against a heated bulk material filter layer (9) of lump coal and/or ceramic pieces, a substantial proportion of the particles being retained on and in the filter layer and subjected to finishing reducing by the reducing gas. A high-temperature flame is produced in front of the filter layer (9) by an oxygen-bearing gas being blown against the filter layer, and the metallised particles which are retained in the filter layer are melted. They pass in the liquid condition through the filter layer (9) into a receiving space (3) for liquid metal (4).

The invention relates to a process as set forth in the classifyingportion of claim 1 and a reducing and smelting furnace as set forth inthe classifying portion of claim 13.

A process of that kind is known from DE-A-21 32 150. In that process,the fine ore which has been subjected to pre-reduction in a pre-reducingcyclone is subjected to finishing reduction in a finishing reducingcyclone by means of the hot waste gases, essentially consisting ofcarbon monoxide, from a smelting and refining installation, and then fedas a solid discharge to the smelting and refining installation. In thatinstallation, the solid discharge is melted down, with the simultaneousproduction of the reducing gas, by incomplete combustion of carbon inexcess, and then refined by means of oxygen.

The reduction rate in the finishing reducing cyclone essentially dependson temperature. Because of what is known as the `sticking effect`, thatis to say the tendency on the part of fine-grain to dust-form spongeiron to stick together at temperatures above 850° C. however, it is notpossible to set the optimum reducing conditions in the finishingreducing cyclone so that hitherto it is not yet technically economicallypossible to achieve the procedure which is basically ideal, namely highlevel of metallisation of the fine ore by means of reducing gas at atemperature above 850° C. and subsequent smelting.

In what are known as smelting reducing processes for fine ore, fine oreis reduced in the molten condition. For that purpose, the processdisclosed in EP-B1-0 063 924 provides that highly heated air or airwhich is enriched with oxygen is injected into the lower region of asmelting and reducing furnace which contains a column of coke, wherebyhigh-temperature zones of between 2000° and 2500° C. are formed in frontof the nozzles. Disposed above the injection nozzles are further nozzlesthrough which hot pre-reduced fine ore is injected by means of hot airor oxygen. In that situation, the pre-reduced fine ore is firstlyoxidised and then melted by the reaction heat in order then to undergofinishing reduction as it sinks down through the bed of coke incounterflow relationship to the hot upwardly moving gases from thehigh-temperature zone, and to be collected in the liquid condition inthe region of the floor. The supply of heat in the lower region of thefurnace must be sufficient to prevent unacceptable cooling of theliquefied iron oxide upon reduction during the downward movement in thesolids bed.

Pre-reduction of the fine ore takes place in a separate pre-reducingstage or in a pre-reducing stage which is integrated in the smelting andreducing furnace, with the waste gas from the smelting furnace beingused as the reducing gas.

The object of the present invent ion is to improve the level ofefficiency in a process of the kind set forth in the opening part ofthis specification, and to permit solids reduction of fine ore abovetemperatures at which the `sticking effect` occurs.

The invention further seeks to provide that the total consumption ofenergy is reduced and excess gas is reduced or eliminated.

The invention further seeks to provide a reducing and smelting furnacefor carrying out the process, which is distinguished by a low level ofconsumption of refractory material for the furnace lining.

The process according to the invention is characterised by the featuresof claim 1 while the apparatus according to the invention ischaracterised by the features of claim 13. Advantageous configurationsof the process according to the invention and the apparatus according tothe invention are set forth in the other claims.

In the process according to the invention, the fine-grain to dust-formmetal oxide particles which are blown with hot reducing gas into a finalreducing space are retained on a heated bulk material filter layerthrough which is passed the gas loaded with the particles, and undergofinishing reduction by the reducing gas flowing therepast. In thatsituation the `sticking effect` is deliberately tolerated so that it ispossible to operate with a reducing gas above 850° C., preferably around950 ° C., at which the reducing reaction takes place more quickly. Thepressure drop at the filter increases due to the material which isdeposited on and clings to the filter and which is here reduced to formsponge iron. Then, by blowing an oxygen-bearing gas, preferably mixedwith coal dust, against the filter layer, the material clinging theretois caused to melt, and passes in the fluid condition through the filterlayer and is received by a receiving space for liquid metal, which isdisposed beneath the filter layer. In that procedure the filter layer isheated above the liquidus temperature of the metal and simultaneouslycleaned.

Preferably, before being injected into the final reducing space, themetal oxide particles are heated and pre-reduced in a pre-reducing spaceof a pre-reducing stage by hot reducing gases in a fluidised bed. Thereducing gas used is preferably at least a part of the waste gas fromthe final reducing space, which has flowed through the filter layer,and/or a part of the waste gas from the receiving space for the liquidmetal. It is sufficient if the metal oxide particles are reduced to aresidual oxygen content of about 50% in the pre-reducing stage, in whichcase it is to be noted here that the temperature is kept below the limitat which a noticeable `sticking effect` occurs. Preferably the metaloxide particles are heated to between 750° and 850° C. in theprereducing stage.

In order to bring the liquid metal in the receiving space to the tappingtemperature, it is advantageous for oxygen-bearing gases and fuel,preferably coal in dust form, to be injected above the metal bath. Thefuel may also be formed by a fluid bed of coal which is formed above themetal bath, by coal being fed to the receiving space in the upperregion. That also produces additional reducing gas.

It has been found advantageous for a gas which is produced in a reformerto be used as the reducing gas with which the metal oxide particles areinjected into the final reducing space, in which respect in particularan installation as is described in DE-A-40 28 853 is particularlysuitable as the reformer. In that procedure which is referred to as theENOR-process, reforming of a CO₂ /H₂ -bearing gas is effected with theaddition of a gasification agent ( coal or hydrocarbon) in a reactor towhich the process heat is supplied by heat carrier particles. The heatcarrier particles are heated within a closed circuit in a heater bycombustion gases which are produced in a combustion chamber. For thatpurpose a part of the waste gas from the final reducing space, thereceiving space for the liquid metal or the pre-reducing cyclone is fedto the combustion chamber, where it undergoes combustion with air.

The bulk material filter layer which is preferably between 20 and 50 cmin thickness can be formed from solid carbonaceous materials such ascoke or refractory materials such as ceramic pieces, or a mixture ofboth. The essential considerations are gas-permeability and heatresistance of the filter layer. The filter layer can be formed on orbetween support grids and the applied material of the filter layer canbe replaced by a periodic or continuous supply of fresh material.

Preferably, the final reducing space is defined by a filter layer bothin an upper region and in a lower region. By virtue of that arrangement,on the one hand the lining of the reducing and smelting furnace isprotected from the direct action of the high-temperature flame whenmelting the sponge iron, while on the other hand there is a simplepossibility of supplementing the consumed material of the filter layerin the lower region by material from the filter layer in the upperregion, to which fresh material is in turn supplied from above. In thecase of a filter layer which contains carbonaceous material in lumpform, coarse-grain or lump-form metal oxide particles may also be addedto the filter layer, such particles being reduced and melted in thelower region, that is to say in the region which is exposed to thehigh-temperature flame.

The blast of oxygen-bearing gas against the filter layer may occur bothin counter-flow relationship and also in co-flow relationship with theliquid metal which passes through the filter layer, that is to say, thehigh-temperature flame can be blown against the filter layer from belowor from above. Preferably the waste gases from the high-temperatureflare are passed through the filter layer in the same direction as theliquefied metal, that is to say the high-temperature flame is directedagainst the top side of the filter layer and the waste gases flowthrough the filter layer in the same direction as the reducing gas andthe liquid metal. In that respect, the oxygen-bearing gas may beinjected into the final reducing space either periodically alternatelywith reducing gas which is charged with the metal oxide particles, orcontinuously.

A reducing and smelting furnace which is suitable for carrying out theprocess includes an upper reducing space and a lower receiving spacewith a partition wall which is partially formed by a bulk materialfilter layer which is held by a support grid and which comprises coal inlump form, in particular coke, and/or ceramic pieces, and whichrepresents a gas and material passage between the final reducing spaceand the receiving space.

Preferably the partition wall between the two spaces is of aconfiguration such as to converge downwardly.

In accordance with a further configuration of the invention, adjoiningthe partition wall between the two spaces, in an upward direction, is afurther curved partition wall which laterally delimits the reducingspace relative to an annular gas space, and which is also at leastpartially formed by a bulk material filter layer supported by a supportgrid. A charging opening at the upper edge of the partition wall permitsrefilling of material for the bulk material filter layer both in theupper region and in the lower region. The partition wall screens therefractory lining of the furnace vessel in the region of the finalreducing space, relative to the high-temperature flame.

The invention is described in greater detail by means of two embodimentswith reference to three diagrammatic Figures of drawings in which:

FIG. 1 shows a process diagram for the reducing phase in a cyclicallyoperated reducing and smelting process,

FIG. 2 shows the smelting and cleaning phase of that process, and

FIG. 3 shows a continuous process with another configuration of thereducing and smelting furnace.

In the process diagrams shown in FIGS. 1 and 2, a reducing and smeltingfurnace 1 has an upper final reducing space 2 and a lower receivingspace 3 for liquid metal 4. The upper final reducing space 2 has aninlet 5 through which fine-grain metal oxide particles and reducing gascan be injected into the final reducing space 2. For that purpose,besides an upper tube 6, there is a coaxial inner tube 7 which extendstowards the inlet 5 only over a part of the length of the outer tube 6and the reducing gas is fed to the annular space between the outer tube6 and the inner tube 7 and the fine-grain metal oxide particles are fedto the inner tube 7.

The final reducing space 2 is separated from the receiving space 3 by apartition wall 8 which is formed by a bulk material filter layer 9 ofcoal in lump or piece form, in particular coke, and/or ceramic pieces,and represents a gas and material passage between the final reducingspace 2 and the receiving space 3. The material of the bulk materialfilter layer is held by a fluid-cooled support grid 10. The grid is ofsuch a configuration as to converge downwardly between the two spaces.In that way, a filter layer 9 of almost uniform thickness can be formedand maintained in the region of the partition wall 8 by virtue of theangle of repose of the material of the filter layer, without an upperboundary provided by a support grid.

The receiving space 3 has a tap opening 11 for the liquid metal 4 and atap opening 12 for slag 13 which floats on the liquid metal 4. In theupper region the receiving space 3 also has a plurality of gas outlets14 which are arranged in a distributed array around the periphery andwhich open into a collecting conduit 15 by way of which waste gas can beremoved from the receiving space 3 by means of a conduit 16.

At least one nozzle 17 for injecting an oxygen-bearing gas, possiblymixed with fine coal, opens in the lower region, immediately above themaximum level of the surface of the bath. Disposed thereabove in theside wall of the receiving space is a charging opening 18 forcarbonaceous material, by way of which coal in lump form can be chargedinto the receiving space in order there to form a fluid bed of coal 19above the layer of slag 13.

In the reducing and smelting furnace shown in FIGS. 1 and 2, adjoiningthe partition wall 8 between the two spaces 2 and 3 in an upwarddirection is a further curved partition wall 20 which laterally delimitsthe final reducing space 2 relative to an annular gas space 21. Like thelower partition wall 8, the partition wall 20 is formed by a bulkmaterial filter layer 9 which is held by a support grid 22 and whichcomprises lump coal, in particular coke, and/or ceramic pieces, blendinginto the bulk material filter layer 9 of the partition wall 8 withoutany interruption. The partition wall 20 is of a configuration such as toconverge upwardly so that, if the angle of repose of the material of thefilter layer 9 is the same as the upper cone angle of the partitionwall, the desired thickness of the filter layer 9 can be maintained,even in the upper region, without an external support grid 23 which ispresent in the illustrated embodiment.

Distributed around its periphery the annular gas space 2 1 has aplurality of openings 24 which are connected to a collecting conduit 25which in turn communicates with gas conduits 26 and 27 for introducing agas into the annular space 21 and for removing the gas therefrom. Theannular gas space 21 is separated from the receiving space 3 by anannular partition wall 46; gas passages (not shown) which can be closedoff can be provided in the partition wall 46 in order to be able to makea communication between the annular gas space 21 and the receiving space3. The controllable gas passages could also be provided by by-passconduits, which can be shut off, between the two above-mentioned spaces.

At the top the partition wall 20 includes an annular charging opening 28for the material of the bulk material filter layer 9. The material canbe fed to that charging opening through a plurality of drop pipes 29 byway of a central intake opening 30. Extending through the annularpartition wall 20, distributed around the periphery thereof, are aplurality of nozzles 31 which are directed towards the inside of thelower partition wall 8. The nozzles could also be passed through thelower partition wall. Oxygen-bearing gas and possibly coal can be blownthrough those nozzles against the lower filter layer 9 in order there toproduce a high-temperature flame.

The fine-grain metal oxide particles which can be fed to the finalreducing space 2 by way of the inner tube 7 are preferably pre-reduced.For that purpose, there is provided a pre-reducing cyclone 32 to whichfine-grain metal oxide particles, in particular iron oxide particles,can be fed by means of the waste gas which is removed by way of theconduit 27, from the annular gas space 21--it could also be anothercarrier gas. The metal oxide particles are supplied, possibly togetherwith coal in powder form, by way of a supply conduit 33 which isconnected to the waste gas conduit 27. The pre-reduced metal oxideparticles leave the pre-reducing cyclone 32 through a lower outlet whichcommunicates with the inner tube 7 by way of a conduit 34. The waste gasfrom the pre-reducing cyclone 32 is removed by way of an upper gasoutlet which communicates through a gas conduit 35 with a gas reformer36 or a gas conduit 37 with a waste gas chimney. The gas reformer 36 canreceive selectively or jointly the waste gas from the pre-reducingcyclone 32, by way of the gas conduit 35, and the waste gas from thereceiving space 3, by way of the gas conduit 16. The energy required forthe gas reforming operation is also obtained from the waste gas eitherfrom the pre-reducing cyclone 32 or from the receiving space 3. For thatpurpose, a part of the gas flow in the conduit 35 and/or the conduit 16is branched off, burnt with air in a combustion chamber of the reformer36 and then passed to the waste gas chimney. That arrangement is notshown in the drawings, for the sake of enhanced clarity thereof.

The gas which is reformed in the gas reformer can be passed selectivelyby way of a conduit 38 into the annular space between the outer tube 6and the inner tube 7 of the reducing and smelting furnace 1, or it canbe passed by way of the conduit 26 into the collecting conduit 25. Theabove-described feed of pre-reduced metal oxide particles through theinner tube 7 and hot reducing gas 38 which is prepared in the gasreformer 36, by way of the annular space between the inner tube 7 andthe outer tube 6 provides a flow of material which is enclosed byreducing gas and which is directed against the filter layer 9 of thelower partition wall 8 so that there is a preferred accumulation ofmaterial in that region of the final reducing space 2. Theabove-mentioned conduits for material and gas include conventionalshut-off members 39-45. Moreover the installation includes furtherpieces of equipment (not shown) such as dust removers, gas scrubbers,blowers, heat exchangers etc.

The performance of a cyclic process will now be described with referenceto FIGS. 1 and 2, FIG. 1 showing the reducing phase and FIG. 2 showingthe smelting and cleaning phase. The conduits which are switched intothe active condition are respectively shown in bold.

In the reducing phase, fine ore mixed with coal is fed to theprereducing cyclone 32 by way of the conduit 33 and waste gas from thefinal reducing space 2 is supplied by way of the conduit 27. The fineore is preheated to about 850° C. in the pre-reducing cyclone 32 andpre-reduced to a residual oxygen content of about 55%. Instead of thepre-reducing cyclone 32, it is also possible to use a plurality ofcyclones, conventional circulating fluidised beds or other knownapparatuses for pre-reduction of fine ore.

The pre-reduced fine ore is blown by way of the inner tube 7 togetherwith fresh reducing gas through the inlet 5 into the final reducingspace 2. The fresh reducing gas is supplied from the gas reformer 36 byway of the conduit 38 and is at a temperature of about 950° C. and is ofa composition of CO+H₂ >90%. Although for reasons of reaction speed thegas temperature should lie above the temperature at which a substantial`sticking effect` occurs in the final reducing space, the process canalso be used with advantage at lower gas temperatures. In comparisonwith fine ore reduction in fluidised beds or cyclones, the fine-grainparticles which are deposited on the filter layer can be exposed, moreintensively and for a longer period of time, to the hot reducing gaseswhich flow through the filter layer.

The gas which is loaded with the pre-reduced fine ore impinges on thepreviously heated filter layer 9 of the partition wall 8, which issupported on the support grid 10. While the gas passes through thefilter layer, the fine-grain particles remain clinging thereto, byvirtue of their sticking tendency, and as a result can be held for asufficiently long period of time in contact with the reducing gas whichis not only drawn off through the lower filter layer 9 of the partitionwall 8 into the receiving space 3 and from there into the collectingconduit 15, but also by way of the upper filter layer 9 of the partitionwall 20 into the annular gas space 21 and from there into the collectingconduit 25. The Gas which is drawn off through the collecting conduit 15passes to the reformer 36 by way of the gas conduit 16 and is thereprocessed to provide the reducing gas which is supplied by way of theconduit 38. The gas which is taken off by way of the collecting conduit25 passes to the pre-reducing cyclone by way of the gas conduit 27. Thewaste gas from the cyclone can additionally be fed to the gas reformer36 by way of the gas conduit 35 or to the waste gas chimney by way ofthe conduit 37.

When, by virtue of the filter layer 9 becoming clogged with the fullyreduced fine ore, the pressure drop in respect of the gases taken fromthe reducing and smelting furnace has reached a predetermined limit,that reducing phase is followed by the smelting and cleaning phase whichis shown in the process diagram illustrated in FIG. 2.

In this phase, cold or preheated oxygen-bearing gas, preferably togetherwith a fuel such as coal dust, is blown through the nozzles 31 againstthe filter layer 9 of the partition wall 8 and in that way ahigh-temperature flame is produced at a temperature of between 2000° and2500° C., by which the metallised particles which are retained on and inthe filter layer are melted and pass in the liquid condition through thefilter layer into the receiving space 3. At the same time the filterlayer is heated above the liquidus temperature of the metal and cleaned.In the illustrated embodiment, during this phase, the feed ofpre-reduced material to the inner tube 7 and the injection of reducinggas by way of the conduit 38 are interrupted and instead the reducinggas from the gas reformer 36, to which the gas from the receiving space3 continues to be supplied by way of the conduit 16, is now injectedinto the annular gas space 21 by means of the conduit 26. By reversal ofthe flow of gas through the filter layer 9 of the upper partition wall20, the filter layer is also cleaned in that region, in other words, anypre-reduced fine ore which is still in a loose condition and which isclinging thereto is conveyed into the final reducing space 2 where itpasses into the smelting region, upon the discharge of gas through thefilter layer 9 of the lower partition wall 8.

The smelting operation is effected in part by virtue of the radiation ofthe high-temperature flame and in part by virtue of convection with thesubsequent flow of the hot waste gases through the filter layer. Withsurplus carbon from injected coal and coke in the filter layer, theprocedure involves substantial reduction of CO₂ +H₂ O to CO+H₂, with areduction in temperature of the gases which are taken off by way of thecollecting conduit 15. As the initially highly heated gas from thehigh-temperature flame which causes liquefaction of the iron on thefilter layer is cooled down again in the coke of the filter layer as aresult of the Boudouard reaction, heat losses can be kept at a low leveland the degree of thermal efficiency can be enhanced.

The material of the filter layer is subjected to consumption in theregion of the partition wall--coke in the filter layer is consumed bythe Boudouard reaction and ceramic pieces are melted--so that the filterlayer must be either periodically or continuously replaced in the regionof the partition wall 8. In the illustrated embodiment, that is done bythe filter layer 9 being extended upwardly in the region of the curvedpartition wall 20 and continuously supplemented by way of the drop tubes29.

A particular advantage of the reducing and smelting furnace shown inFIGS. 1 and 2 is that the refractory material of the furnace--it is notshown in the diagrammatic Figures of the drawings--is shielded relativeto the high flame temperatures required for the fusing operation, by thefilter layers of the lower partition wall 8 and the curved partitionwall 20. The great amount of heat which occurs in the smelting andcleaning phase in the final reducing space 2 is absorbed by the filterlayers and the water-cooled support grids 10 and 22 respectively, afurther cooling action being produced by the reducing gas which isinjected in the reversal phase by way of the annular gas space 21 in thefilter layer of the upper partition wall 20.

The liquid iron which has passed through the filter layer 9 of thepartition wall 8 in the smelting and cleaning phase drips into thereceiving space 2 where it is collected at the bottom in the form of amolten bath 4. In order to maintain a sufficiently high temperature inthe molten bath or to increase the temperature thereof to the tappingtemperature, oxygen-bearing gas is injected by way of the nozzle 17 anda fluid bed 19 of coal is formed above the injection zone by a feed ofcoal by way of the charging opening 18. In that way additional reducinggas is produced, which is removed by way of the collecting conduit 15and fed to the gas reformer 36.

After the smelting and cleaning phase the filter layer 9 is cleaned andheated, and the reducing phase which was described with reference toFIG. 1 is again effected. For that purpose the shut-off members 39-45are correspondingly switched over.

Although a cyclic process with reducing phase and smelting phase wasdescribed with reference to FIGS. 1 and 2, those two phases, withsuitable matching, could also be combined together to provide acontinuous process. In that case, oxygen-bearing gases and coal areinjected through the nozzles 31 continuously or only with briefinterruptions, and the material which clings to the filter layer 9 ofthe lower partition wall 8 is continuously melted.

FIG. 3 also shows a continuous procedure with a modified embodiment ofthe reducing and smelting furnace. Parts which correspond to those ofthe first embodiment are identified by the same reference numerals andare no longer described separately. In the reducing and smelting furnace100 shown in FIG. 3 the upper partition wall 20 is of a cylindricalconfiguration. The material of the filter layer 9 can be supplied to theannular charging opening 28 in a similar manner as described withreference to the first embodiment. The nozzles 31 for injecting theoxygen-bearing gas into the final reducing space 2 are replaced by acentral tube 131 which at the lower end has a plurality of nozzleopenings which are directed towards the filter layer 9 of the partitionwall 8. In addition, there are a plurality of the injection deviceformed from the tubes 6 and 7 in the first embodiment, for pre-reducedfine ore and hot reducing gas. The coaxial tubes 6 and 7 are arranged ina distributed array around the central tube 131 within the annularcharging opening 28.

In the embodiment shown in FIG. 3, pre-reduced fine ore is blowntogether with hot reducing gas into the final reducing space 2 throughthe tubes 6/7. At the same time oxygen-bearing gas and possibly coal areinjected by way of the central tube 131 and, when that happens, ahigh-temperature flame is produced in the region directly above thefilter layer 9 of the partition wall 8. The gases leave the finalreducing space 2 by way of the filter layer 9 of the lower and lateralpartition walls 8 and 20 and are recycled from the gas outlets 14 and 24by way of the conduits 16 and 26/47 to the gas reformer 36. The upperfilter layer 20 can be occasionally cleaned by reversal of the flow ofgas by way of the conduit 26. The particles which cling to the filterlayer 9 are continuously melted by the high-temperature flame and passthrough the lower filter layer into the receiving space 3.

The reducing and smelting furnace described is also suitable foradditionally reducing and smelting ore in lump form, which is suppliedby way of the charging opening 28, mixed with the material of the filterlayer 9. In this case, as upon reversal of the last-described process,the hot reducing gas from the reformer 36 is not only blown through thetubes 6 but also by way of the conduit 26 and through the inlet 24 intothe annular gas space 21 and then through the cylindrical partition wall20 into the final reducing space 2. The coke-bearing material of thefilter layer 9, which is consumed in the lower region of the filterlayer, is heated as it moves downwardly, and reduced by the hot reducinggas supplied to the annular space 21. When, due to consumption of thefilter layer, the material moves out of the region of the cylindricalpartition wall 20 into the region of the lower partition wall 8, it ismelted due to the heat which is generated there by the high-temperatureflame, and, together with the reduced material from the final reducingspace, it drips into the receiving space 3. In the described embodimentsthe filter layer is adjusted to a thickness of between 20 and 50 cm inthe lower region, that is to say in the region of the partition wall 8.

I claim:
 1. A process for the production of liquid metal from metaloxide particles, wherein the metal oxide particles are blown togetherwith hot reducing gas into a final reducing space of a final reducingstage, said process comprising the steps of passing the hot reducing gaswhich is loaded with the particles through a heated bulk material filterlayer of solid lump form material, retaining a substantial proportion ofthe particles on and in the filter layer, finishing reducing theparticles by subjecting the particles to the reducing gas, a flame infront: of the filter layer by blowing an oxygen-bearing gas against thefilter layer, to melt the metallised particles which are retained on andin the filter layer, to pass the melted particles in the liquidcondition through the filter layer and into a receiving space for liquidmetal and at the same time heating the filter layer above the liquidustemperature of the metal to clean the filter layer.
 2. A process as setforth in claim 1 comprising the further step that prior to beinginjected into the final reducing space the metal oxide particles areheated and pre-reduced in a pre-reducing space of a pre-reducing stageby reducing hot gases in a fluidised bed.
 3. A process as set forth inclaim 2 wherein the pre-reducing step includes reducing the metal oxideparticles in the pre-reducing stage to a residual oxygen content ofbetween 40 and 60%.
 4. A process as set forth in claim 2 wherein thepre-reducing step includes heating the metal oxide particles in thepre-reducing stage to between 750° and 850° C.
 5. A process as set forthin claim 1 wherein the step of finishing reducing the particles includesinjecting into the final reducing space reducing gas which is at atemperature of between 850° and 950° C.
 6. A process as set forth inclaim 1 including the further step of forming a fluid bed of coal in thereceiving space for the liquid metal by injecting oxygen-bearing gasesabove the metal bath, and feeding coal into the receiving space.
 7. Aprocess as set forth in claim 2 including the further step of feedingthe hot waste gases from the receiving space for the liquid metal to thepre-reducing space for producing the reducing gas.
 8. A process as setforth in claim 2 including the further step of feeding the waste gasesof the pre-reducing space to a gas reformer for producing the reducinggas.
 9. A process as set forth in claim 1 wherein the step of passing agas through a filter layer includes the steps of providing the filterlayer with an upper and a lower region to define a final reducing spacetherein, introducing reducing gas into an upper portion of the finalreducing space and passing the reducing gas through the lower region ofthe filter layer.
 10. A process as set forth in claim 9 wherein the stepof introducing the reducing gas includes periodically alternatelyinjecting reducing gas loaded with pre-reduced metal oxide particles andoxygen-bearing gas into the final reducing space.
 11. A process as setforth in claim 1 including the further steps of providing the bulkmaterial filter layer on or between support grids and replacing consumedmaterial of the filter layer by a periodic or continuous feed of freshmaterial.
 12. A process as set forth in claim 11 wherein the step ofreplacing consumed material includes providing coarse-grain or lumpmetal oxide particles to the filter layer.
 13. A process according toclaim 1 wherein the step of retaining particles on and in the filterlayer includes the step of providing the filter layer with a layer oflump coal.
 14. A process according to claim 13 wherein the step ofproviding the filter layer with a layer of lump coal includes providingthe filter with a layer of coke.
 15. A process according to claim 1wherein the step of retaining particles on and in the filter layerincludes the step of providing the filter layer with a layer of ceramicpieces.
 16. A process according to claim 1 wherein the step of passing agas loaded with particles includes passing a gas loaded with iron oxideparticles.